The present invention relates to a dual channel optical mapping apparatus and to methods which can be used to supply images from essentially transparent objects or tissue using different depth resolutions, or, sequentially, images with adjustable depth resolution at the same or at different wavelengths, as required to observe fluorescence or Raman radiation emitted by the object. The two channels of the dual channel apparatus could be either a confocal channel and an optical coherence tomography channel, two optical coherence tomography channels, or two confocal channels.
In the description which follows, reference is made primarily to the eye as the object. This has to be understood as merely a way to help the description and not as a restriction of the application of the present invention. As such, where the term xe2x80x9ceyexe2x80x9d is used, a more general transparent and scattering object or organ may be sought instead.
High depth resolution imaging of the eye fundus can be achieved by optical coherence tomography (OCT) as shown in the paper xe2x80x9cOptical coherence tomographyxe2x80x9d by D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee. T. Flotte, K. Gregory, C. A. Puliafito and J. G. Fujimoto, Science 254, (1991), pp. 1178 and in the paper xe2x80x9cOptical coherence tomographyxe2x80x9d by A. F. Fercher, in J. Biomed. Opt., 1(2), (1996), pp. 157-173. OCT has the potential of achieving much better depth resolution, as the limit in this case is not set by the eye, but by the coherence length of the source. For example, optical sources, such as superluminiscent diodes and mode-locked lasers are now available with coherence lengths below 20 xcexcm.
An OCT apparatus is now commercially available (e.g. from Humphery), which produces longitudinal images only, i.e. images in the planes (x,z) or (y,z), where the z axis is perpendicular to the patient""s face and x and y axes are in the plane of the patient""s face. Examples of such apparatus for longitudinal imaging are described in U.S. Pat. Nos. 5,493,109, 5,537,162, 5,491,524, 5,469,261, 5,321,501 and 5,459,570.
OCT has also been reported as being capable of providing en-face (or transversal) images, as reported in xe2x80x9cCoherence Imaging by Use of a Newton Rings Sampling Function⇄ by A. Gh. Podoleanu, G. M. Dobre, D. J. Webb, D. A. Jackson, published in Opt. Lett., Vol. 21, No. 21, (1996), pp. 1789-1791, xe2x80x9cSimultaneous En-face Imaging of Two Layers in Human Retinaxe2x80x9d Opt. Letters, by A. Gh. Podoleanu, G. M. Dobre, D. J. Webb, D. A. Jackson, published in Opt. Lett., 1997, vol.22, No. 13, pp. pp. 1039-1041 and xe2x80x9cEn-face Coherence Imaging Using Galvanometer Scanner Modulationxe2x80x9d by A. Gh. Podolenu, G. M. Dobre, D. A. Jackson, Opt. Lett. 23, pp. 147-149, 1998. When applied to the eye, however, the en-face OCT images look fragmented, as demonstrated in xe2x80x9cTransversal and Longitudinal Images from the Retina of the living Eye Using Low Coherence Reflectometryxe2x80x9d, by A. Gh. Podoleanu, Mauritius Seeger, George M. Dobre, David J. Webb, David A. Jackson and F. Fitzke, published in the Journal of Biomedical Optics, 3(1), pp. 12-20, 1998. These papers also demonstrate that, owing to the low coherence length, the OCT transversal images show only fragments of the retina and are difficult to interpret.
To improve the usefulness of en-face OCT images, a dual presentation of images was proposed as described in U.S. Pat. No. 5,975,697. Two en-face images are produced and displayed simultaneously; one of which is an OCT image and the other is a confocal image (similar to the image produced by a scanning laser ophthalmoscope (SLO)). The dual presentation allows the fragments sampled by the OCT of the fundus to be uniquely placed in correspondence with fundus images displayed by the confocal channel. The confocal channel, however, has a much larger depth resolution and the images look continuous, offering good guidance of the part of the eye investigated.
The dual display is generally essential not only for guidance, but for subsequent alignment and processing of the stack of en-face images prior to reconstruction of the 3D volume investigated. In addition, it enables the same location in the eye to be accessed easier on subsequent examinations. However, a practical problem of the dual channel imaging instruments is that focusing has to be adjusted simultaneously for both channels. No provisions were presented in U.S. Pat. No. 5,975,697 with respect to this feature.
Another problem with the prior art technique is that the confocal channel taps some of the possibly already weak return signal from the tissue, which results in a lower signal to noise ratio in the OCT channel. For instance, when 10% of the signal is tapped, the loss in the OCT channel can be more than 19%. Therefore, it would be desirable, especially when the target returns a weak signal, to eliminate confocal tapping and return all of the signal to the OCT channel. On the other hand, there are situations when the presentation of an OCT image is not needed and depth analysis using the confocal channel only may be required. Unfortunately, however, the beam-splitter ratio in U.S. Pat. No. 5,975,697 is fixed and therefore, such versatility cannot be achieved.
Another problem is that when this technique is used for OCT of skin or teeth, longer wavelengths are recommended for providing better penetration depth. However, the gain of photocathodes and avalanche photodetectors at longer optical wavelengths is much poorer than that for visible light or, for example, the 800 nm band which is preferred for the retina. Therefore, at longer wavelengths, poorer performance of the confocal channel of the dual instrument as presented in U.S. Pat. No. 5,975,697 is expected. Another problem with the dual channel imaging instrument as described in U.S. Pat. No. 5,975,697 is that the wavelengths of the two channels are the same. The system as such cannot be used to generate a confocal image at a different wavelength from that used in the OCT channel. This prevents the utilisation of the system in fluorescence and autofluorescence imaging, or for Raman studies.
In U.S. Pat. No. 5,459,570, the beam-splitter shared by the confocal and the OCT channel is used in transmission. It is known that the dispersion of the optical material used in beam-splitters, if left uncompensated, leads to deterioration of the depth resolution in the OCT channel. An on-axis fixation lamp is also required for the investigation of the fovea. This requires other beam-splitters to be introduced in the system, which adds further dispersion to the OCT channel.
A still further problem with en-face scanning using galvanometer scanners is that the fly-back of the galvanometer scanner is finite and consequently, more than 20% of the period time of the ramp signal driving the galvanometer scanners, at kHz rates, may be wasted.
In terms of transverse resolution, this feature depends on how well the focus is matched to the coherence position (wherein tracking of the focusing and zero optical path difference are referred to as dynamic focus). Dynamic focus was described in PCT patent publication No. WO 92/19930, but only in principle. Possible optical configurations to simultaneously scan the depth and the position of the focus in the depth are described in U.S. Pat. No. 4,589,773 and in U.S. Pat. No. 6,057,920. These solutions however, require mechanical synchronism of elements or adjustment of ratios of focal lenses. The prior art method works only when the index of refraction of the tissue is known. If the tissue consists of layers of different index of refraction, different adjustments are required. The methods described are devised especially for longitudinal OCT, where B-scan images are generated by fast scanning along the depth coordinate with a slower scanning along a transverse coordinate. As such, the method needs to be fast, and operational at the depth scanning rate of, for example, a rate on the order of 100-1000 Hz. Once different solutions have been devised as described in U.S. Pat. No. 6,057,920, it is very difficult to reconfigure the technique for different values of the refractive index or to apply corrections for multiple layers of different index of refraction.
Another mechanical configuration is disclosed in U.S. Pat. No. 6,201,608, however this method is not applicable for use with the eye.
U.S. Pat. No. 6,172,752B1 discloses a low coherence interferometer where the thickness and the index of refraction of a transparent plate can be determined by measuring the displacement of the plate, or of the lens in the front of the plate in the object arm and of the mirror in the reference arm. The method uses these displacements and the numerical aperture of the beam entering the sample. However, when imaging different layers in the scattering tissue, the index of refraction is unknown and the method is not applicable as no tracking mechanism of the two displacements is described. Additionally, when imaging tissue and the eye, the interface optics complicates the equation which relates the two displacements to be tracked, and additionally, the patients have different eye lengths (i.e. the numerical aperture is not known and the method again is not applicable).
Thus, a need exists for better procedures of implementing dual channel imaging with different depth resolution, which procedures can allow imaging at different wavelengths and allow for focusing adjustment to be tracked in both channels while being compatible with dynamic focus in order to maintain both channels in focus. In particular, better procedures having improved efficiency and which make better use of scanning devices and of the sensitivity of photodetectors, and which can allow for versatile operation of the two channels to cover a large range of possible imaging regimes with the same hardware, are desirable. Accordingly, the present invention provides for improvements over at least one of the problems of the prior art as stated hereinabove, or as described hereinbelow.
As a first advantage, the present invention sets out to solve the above discussed problems and relates to an apparatus wherein both receiver apertures (e.g. OCT and confocal) are maintained in focus.
In a second advantage, the present invention sets out methods and apparatuses to ensure that the confocal optical receiver operates at the maximum sensitivity.
In a third advantage, the present invention sets out methods and apparatuses which allow:
(i) switching from a dual OCT/confocal regime to a predominantly single channel operation regime of either OCT only, or of a predominantly confocal regime (with only minor sensitivity loss to the OCT channel), wherein by balancing the signals returned from the object to the OCT and confocal channels either: (a) complete extinction of the signal returned from the object towards the confocal channel is possible; or (b) a significant reduction of the signal returned to the OCT channel is possible while maximising the signal returned to the confocal channel;
(ii) or operation in a mode having two confocal channels imaging at two different wavelengths.
In a fourth advantage, the invention sets out methods and apparatuses to quasi-simultaneously produce two images of different depth resolution from the same depth.
In a fifth advantage, the invention sets out methods and apparatuses to quasi-simultaneously produce two images at two different wavelengths, as required for the observation of auto-fluorescence or fluorescence, or Raman in the object under investigation.
In a sixth advantage, the invention sets out methods and apparatuses to generate pairs of images or an image, where the pixel position is determined within the raster (or image display) by the actual coordinates of the transversal scanners, irrespective of the movement direction of the transversal scanners. Alternatively, the invention sets out methods and apparatuses to generate double pairs of images for each direction of movement of the transversal scanners.
In a seventh advantage, the invention sets out methods and apparatuses to process and eventually superimpose the OCT image on to the confocal image, of the same or different wavelength, or to produce, process and eventually superimpose OCT images of different depth resolutions, from the same depth.
In an eighth advantage, the invention sets out methods and apparatuses to track the focus in the OCT and confocal channel with the depth scanned in the OCT channel.
The advantages set out hereinabove, as well as other objects and goals inherent thereto, are at least partially or fully provided by an improved optical mapping apparatus of the present invention, having improved focusing means, a versatile optical splitter shared by the confocal and OCT channel and switched optical sources synchronised with imaging means.
Accordingly, in a first aspect, the present invention provides an optical mapping apparatus which comprises:
an optical coherence tomography (OCT) system built around an in-fiber or a bulk interferometer excited by an optical radiation source;
a confocal optical receiver with or without adjustable depth resolution;
an optical splitter, shared by both the interferometer of the OCT and the confocal optical receiver, to direct some of the light returned from an object situated at the object location to the optical confocal optical receiver, where the OCT channel uses the optical-splitter in reflection and the confocal channel in transmission, xe2x80x94regime called as xe2x80x9cReflectionxe2x80x94OCT/Transmissionxe2x80x94Confocalxe2x80x9d, hereinafter xe2x80x9cR-OCT/T-Cxe2x80x9d or optionally wherein the OCT channel uses the optical-splitter in transmission and the confocal channel uses the optical splitter in reflection, xe2x80x94regime called xe2x80x9cTransmission OCT/Reflection-Confocalxe2x80x9d, hereinafter xe2x80x9cT-OCT/R-Cxe2x80x9d;
transverse scanning means, preferably consisting of a line scanner and a frame scanner, to effect transverse scanning of an optical output from the optical splitter (as an imaging beam), over a line or a predetermined area in the object;
interface optics for transferring an optical beam from the transverse scanning means to the object and for transferring an optical output beam reflected and scattered from the object back to the optical-splitter through the transverse scanning means, and from the optical-splitter to both the interferometer of the OCT channel and/or the optical confocal optical receiver of the confocal channel, in a ratio determined by the optical splitter and wavelength;
optionally a fixation lamp for sending light from an external source towards the object;
optionally all interface optics-splitter shared by the fixation lamp beam and the imaging beam, wherein the interface optics-splitter can be used either in reflection or transmission by the imaging beam while respectively the fixation lamp beam is transmitted or reflected, respectively;
focusing adjustment means placed between the optical-splitter and the transverse scanning means, to simultaneously maintain the input aperture of the interferometer and the aperture of the confocal optical receiver in focus, while focusing the scanned beam on the object;
optionally means to introduce intensity or phase modulation or intensity modulation and phase modulation in the said OCT interferometer;
analysing means, coupled to the raster scanning means, for demodulating the photodetected signals of the photodetectors in the interferometer and confocal optical receiver;
optionally depth adjustment means for altering the optical path difference in said OCT interferometer, over a predetermined amount for at least one point in the raster in either steps or continuously at a pace synchronised with the focusing adjustment means, according to a synchronising procedure;
displaying means for processing and generating an image created by the interferometer and an image created by the confocal optical receiver for the simultaneous display of the said respective images created by the interferometer and the confocal optical receiver, which images are synchronised with the transverse scanning means;
optionally timing means which control two main operation regimes, namely (i) en-face imaging when the mapping apparatus acquires transverse images in a plane perpendicular on the optic axis (or in the patient face) at constant depth for different depths and (ii) longitudinal imaging when the mapping apparatus acquires longitudinal images containing the optic axis (or perpendicular to the patient face).
In a preferred feature, the optical mapping apparatus is as described hereinabove wherein said confocal optical receiver is part of a block CE, which consists of at least one confocal optical receiver with or without adjustable depth resolution and optionally, at least one excitation source to excite fluorescence or Raman radiation from the object, whole the aperture of the block CE is optically conjugate to the apertures of the confocal optical receiver and of the excitation source; and wherein:
said optical splitter, shared by both the interferometer of the OCT and the block CE, directs some of the light returned from an object situated at an object location adjacent to the optical mapping apparatus, wherein, the OCT channel uses the optical-splitter in reflection and the block CE in transmission (R-OCT/T-CE), or wherein the OCT channel uses the optical-splitter in transmission and the CE block uses the optical splitter in reflection (R-OCT/R-CE);
said interface optics transfers an optical beam from the transverse scanning means to the object, and an optical output beam reflected and scattered from the object back to the optical-splitter through the transverse scanning means, and, from the optical-splitter to the interferometer of the OCT channel and to the block CE in a selected ratio, which ratio is determined by the optical splitter used and the wavelength of the radiation backscattered or emitted by the object; and
said focusing adjustment means is placed between the optical-splitter and the transverse scanning means, to simultaneously maintain the input aperture of the interferometer and the aperture of the CE block in focus, while focusing the scanned beam on the object.
Further, the optical mapping apparatus is preferably one wherein said optical radiation source is made out of two optical sources of different wavelengths which are combined by a fiber directional single mode coupler or a bulk beam-splitter;
said depth adjustment means alters the focus, over a predetermined amount for at least one point in a raster in either steps or continuously at a pace synchronised with the focusing adjustment, according to a synchronising procedure;
said displaying means for generating and processing the images created by the confocal optical receivers, is synchronised with the transverse scanning means, wherein a line in the image corresponds to the line scanner movement and the advance of said line to the completion of the area scanned corresponds to the movement of the frame scanner; and
said timing means controls the 3D scanning operation regime, when the mapping apparatus acquires en-face images in a plane perpendicular on the optic axis (or in the patient face) at different focusing depths.
In a further preferred feature, the transverse scanning means comprises a line scanner and a frame scanner, and still further, wherein a line in an object corresponds to the line scanner movement and the advance of the line to the completion of the area scanned corresponds to the movement of frame scanner. Still more preferably, the analysing means is coupled to the transverse scanning means.
The optical radiation source is preferably a low coherence source, or a source with adjustable coherence length.
Further, it is preferred that the depth adjustment means and the focusing adjustment means use synchronised PC controlling means, with independent initial position, velocity and acceleration and deceleration, which can be controlled continuously or in a stepwise manner.
Further, in one preferred embodiment, the present invention provides an optical mapping apparatus which comprises:
an optical coherence tomography (OCT) system built around an in-fiber or a bulk interferometer excited by an optical radiation source;
transverse scanning means consisting from a line scanner and a frame scanner to effect transverse scanning of the object using an optical output from the optical splitter (as an imaging beam), over a line or a predetermined area in the object;
interface optics for transferring an optical beam from the transverse scanning means to the object, and for transferring an optical output beam reflected and scattered from the object back to the OCT system;
optionally a fixation lamp for sending light from an external source towards the object;
optionally, an interface optics-splitter shared by the optional fixation lamp beam and the imaging beam, wherein the interface optics-splitter can be used either in reflection or transmission by the imaging beam, while the fixation lamp beam is transmitted or reflected, respectively;
focusing adjustment means placed between the output of the OCT system and the transverse scanning means, to maintain the input aperture of the interferometer in focus, while focusing the scanned beam on the object;
optionally means to introduce intensity or phase modulation or intensity modulation and phase modulation in the OCT interferometer;
analysing means, coupled to the transverse scanning means, for demodulating the photodetected signals of the photodetectors in the interferometer;
depth adjustment means for altering the optical path difference in said OCT interferometer over a predetermined amount for at least one point in the transverse scanning means in either steps or continuously at a pace synchronised with the focusing adjustment means, according to a synchronising procedure;
displaying means for generating and processing the image created by the interferometer synchronised with the transverse scanning means, where the line in the image corresponds to the line scanner movement and the advance of the line to the completion of the area scanned corresponds to the movement of the frame scanner;
optionally timing means which control two main operation regimes, namely (i) en-face imaging when the mapping apparatus acquires transverse images in a perpendicular plane at constant depth and (ii) longitudinal imaging when the mapping apparatus acquires longitudinal images in a parallel plane.
In a further aspect, the present invention also provides a method of preparing a dual channel image of an object, which method utilizes an optical mapping apparatus as described hereinabove with respect to the present invention, or as described hereinbelow.
In a still further aspect, the present invention also provides for the use of the apparatus described hereinabove with respect to the present invention.